In cooperation with the Bureau of Reclamation 

Estimation of Monthly Evaporation from Lake Ashtabula


in North Dakota, Orwell Lake in Minnesota, and Lake


Traverse in Minnesota and South Dakota, 1931-2001


Water-Resources Investigations Report 03-4282


N O R T H D A K O T A M I N N E S O T A 

S O U T H D A K O T A 

Beardsley 

Sisseton 

Cooperstown 

Valley 

City Jamestown 

Wheaton 

Campbell 

Fergus 

Falls 

River 

Sheyenne 

River 

James 

Lake 

Ashtabula 

Lake 

Traverse 

of 

Red River 

the 

North 

FOSTER 

EDDY 

NELSON 

TRAILL 

STEELE 

GRIGGS 

BARNES CASS 

LA MOURE RANSOM

STUTSMAN 

DICKEY SARGENT 

RICHLAND 

ROBERTS 

MARSHALL 

NORMAN MAHNOMEN 

CLAY BECKER

OTTER TAIL 

WILKIN 

GRANT DOUGLAS 

STEVENS 

TRAVERSE 

BIG STONE 

Bois de 

Otter 

Tail 

R. 

Sioux R. 

Orwell Lake 

(Reservoir) 

Fargo 

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

U.S. Geological Survey

Estimation of Monthly Evaporation from Lake Ashtabula


in North Dakota, Orwell Lake in Minnesota, and Lake


Traverse in Minnesota and South Dakota, 1931-2001


By Kevin C. Vining 

Water-Resources Investigations Report 03-4282


In cooperation with the Bureau of Reclamation 

Bismarck, North Dakota 

2003 
U.S. DEPARTMENT OF THE INTERIOR 

GALE A. NORTON, Secretary 

U.S. GEOLOGICAL SURVEY

CHARLES G. GROAT, Director 

Any use of trade, product, or firm names is for descriptive purposes only 

and does not imply endorsement by the U.S. Government. 

For additional information write to: 

District Chief 

U.S. Geological Survey

Water Resources Division 

821 East Interstate Avenue 

Bismarck, ND 58503-1199 

Copies of this report can be purchased from: 

U.S. Geological Survey

Information Services 

Box 25286 

Denver, CO 80225-0286 
CONTENTS


Abstract .....................................................................................................................................................................................................1


Introduction ...............................................................................................................................................................................................1


Methods of estimating evaporation ...........................................................................................................................................................1


Data acquisition.........................................................................................................................................................................................3


Estimation of reservoir evaporation ..........................................................................................................................................................3


Method considerations and limitations ...................................................................................................................................................11


Summary .................................................................................................................................................................................................12


References ...............................................................................................................................................................................................13


FIGURES 

1. Map showing location of the Lake Ashtabula, Orwell Lake, and Lake Traverse study sites, climate stations, and


Wetland P1 and Williams Lake energy budget evaporation sites in North Dakota, Minnesota, and South Dakota ..........................2


2. Graph showing monthly evaporation in relation to monthly mean air temperature determined from the energy


budget evaporation site at Wetland P1 in North Dakota, 1982-85 and 1987 .....................................................................................4


3. Graph showing monthly evaporation in relation to monthly mean air temperature determined from the energy


budget evaporation site at Williams Lake in Minnesota, 1982-86 .....................................................................................................4


4. Graph showing annual evaporation determined from the regression equation for Lake Ashtabula in North Dakota,


Orwell Lake in Minnesota, and Lake Traverse in Minnesota and South Dakota, 1931-2001 .........................................................12


TABLES 

1. Distances from reservoir study sites to Wetland P1 in North Dakota and to Williams Lake in Minnesota and


fractional contributions applied to the evaporation data sets from Wetland P1 and Williams Lake to obtain


estimated monthly evaporation from Lake Ashtabula in North Dakota, Orwell Lake in Minnesota, and


Lake Traverse in Minnesota and South Dakota .................................................................................................................................5


2. Estimated monthly evaporation, in centimeters, from Lake Ashtabula in North Dakota, 1931-2001 ...............................................6


3. Estimated monthly evaporation, in centimeters, from Orwell Lake in Minnesota, 1931-2001 .........................................................8


4. Estimated monthly evaporation, in centimeters, from Lake Traverse in Minnesota and South Dakota, 1931-2001.......................10


iii 
Estimation of Monthly Evaporation from Lake Ashtabula in North 

Dakota, Orwell Lake in Minnesota, and Lake Traverse in 

Minnesota and South Dakota, 1931-2001 

By Kevin C. Vining 

Abstract 

Reservoirs on tributaries of the Red River of the North provide water for Fargo and Grand Forks, N. Dak., 

and other cities along the river. Adequate estimates of evaporative losses from the reservoirs are needed to 

determine the total water supply in the Basin. Many equations could be used to estimate lake or reservoir 

evaporation. However, in addition to measurements of air temperature, the equations require measurements of 

net radiation, wind speed, and relative humidity. Evaporation and air temperature data from energy budget 

evaporation sites at Wetland P1 in North Dakota and at Williams Lake in Minnesota are available. Air 

temperature data collected from climate stations near Lake Ashtabula in North Dakota, from Orwell Lake in 

Minnesota, and from Lake Traverse in Minnesota and South Dakota also are available. Therefore, the combined 

data sets were used to estimate monthly evaporation from Lake Ashtabula, Orwell Lake, and Lake Traverse. 

Averaged monthly mean air temperatures determined for each reservoir study site were used to calculate monthly 

evaporation data sets for 1931-2001. Results from the procedure that estimates reservoir evaporation indicate 

that slight downward trends in annual evaporation occurred from 1931-2001. The trends may have been caused 

by the selected time period of the study, which began with the drought conditions in the mid 1930’s and ended 

with the more wet conditions in the late 1990’s. Average annual evaporation values for each reservoir for 1931­

2001 correspond well with published average annual lake evaporation values for 1946-55. 

INTRODUCTION 

Reliable water supplies are needed for population growth of cities in the Red River of the North Basin. Currently, 

reservoirs on tributaries of the Red River of the North provide water for Fargo and Grand Forks, N. Dak., and other cities 

along the river. The Dakota Water Resources Act was passed by the U.S. Congress on December 15, 2000 (Garrison 

Diversion Conservancy District and the Dakotas Area Office of the Bureau of Reclamation, [n.d.]). The Act authorized the 

Secretary of the Interior to conduct a comprehensive study of the future water-quantity and quality needs of the Red River 

of the North Basin in North Dakota and the possible options to meet those water needs. Adequate estimates of evaporative 

losses from the reservoirs in the Red River of the North Basin are needed to determine the total water supply in the Basin. 

To provide the needed evaporation data, the U.S. Geological Survey conducted a study in cooperation with the Bureau of 

Reclamation to review studies that evaluated evaporation estimation methods and to estimate monthly evaporation from 

selected reservoirs for 1931-2001. This report describes the results of the study. The reservoirs for which monthly 

evaporation was estimated are Lake Ashtabula in North Dakota, Orwell Lake in Minnesota, and Lake Traverse in 

Minnesota and South Dakota (fig. 1). 

METHODS OF ESTIMATING EVAPORATION 

Pan evaporation data commonly are used to estimate evaporation from lakes and reservoirs; however, there are 

problems with using these data. Debris in the water, animal activity in and around the pan, size of the pan, materials used 

to construct the pan, exposure of the pan, strong winds, and the measurement of water depth in the pan can introduce errors 

in pan evaporation rates (Farnsworth and others, 1982; Rosenberg and others, 1983). Shallow free-water-surface 

evaporation atlases of the United States have been created by using pan evaporation data, but the values in the atlases 

cannot be used to estimate lake or reservoir evaporation unless heat storage changes and heat inflows and outflows from the 

lakes and reservoirs can be accounted for (Farnsworth and others, 1982). 

1 
M A N I T O B A


EXPLANATION 

1:2,000,000, 1972 

N O R T H D A K O T A M I N N E S O T A 

S O U T H D A K O T A 

97 ̊ 

97 ̊ 

95 ̊ 

95 ̊ 

49 ̊ 

47 ̊ 47

49 ̊ 

99 ̊ 

99 ̊ 

CAVALIER PEMBINA

TOWNER 

BENSON 

WELLS 

PIERCE 

FOSTER 

EDDY SHERIDAN 

RAMSEY WALSH 

NELSON GRAND FORKS 

TRAILL 

STEELE 

GRIGGS 

BARNES CASS 

LA MOURE RANSOM

STUTSMAN 

DICKEY SARGENT 

RICHLAND 

ROBERTS 

MARSHALL 

KITTSON ROSEAU

MARSHALL 

PENNINGTON 

RED LAKE 

CLEARWATER 

POLK 

NORMAN MAHNOMEN 

CLAY BECKER

OTTER TAIL 

WILKIN 

GRANT DOUGLAS 

TODD 

WADENA 

HUBBARD 

CROW WING 

CASS 

MORRISON 

STEVENS 

TRAVERSE 

BIG STONE 

BELTRAMI 

KOOCHICHING 

WETLAND P1 

ENERGY BUDGET 

EVAPORATION SITE 

WILLIAMS LAKE 

ENERGY BUDGET 

EVAPORATION SITE 

Beardsley 

Sisseton 

Cooperstown 

Valley 

City Jamestown 

Wheaton 

Campbell 

Fergus 

Falls 

Upper 

Red 

Lake Devils

 Lake 

River

River 

James 

Sheyenne Stump

 Lake 

Lake 

Ashtabula 

Lower 

Red 

Lake 

Lake 

Winnibigoshish 

(Reservoir) 

Leech Lake 

(Reservoir) 

Mississippi 

Mississippi 

River 

River 

Lake 

Traverse 

RED RIVER OF THE 

0 20 40 60 KILOMETERS 

Red 

River 

of 

the 

North 

Bois de 

Otter 

Tail 

River 

Sioux R. 

Orwell Lake 

(Reservoir) 

Fargo 

Grand 

Forks 

Base from U.S. Geological Survey 

 ̊ 

Red River of the North Basin boundary 

Climate station 

NORTH BASIN 

Reservoir study sites 

NORTH DAKOTA 

SOUTH DAKOTA 

MINNESOTA 

Sisseton Municipality INDEX MAP

Figure 1. Location of the Lake Ashtabula, Orwell Lake, and Lake Traverse study sites, climate stations, and Wetland P1 and


Williams Lake energy budget evaporation sites in North Dakota, Minnesota, and South Dakota.


The energy budget method was used to determine evaporation from a wetland in North Dakota, Wetland P1, and from 

a lake in Minnesota, Williams Lake (Rosenberry and others, 1993; Parkhurst and others, 1998). The energy budget method 

relies on the optimum placement of sensors in and around a lake or reservoir to provide a rigorous estimate of evaporation 

(Rosenberry and others, 1993). In another study, the energy budget method was compared to 11 other evaporation 

equations in order to determine monthly evaporation from Williams Lake (Winter and others, 1995). Of the 11 equations 

evaluated, a modified DeBruin-Keijman equation, the Priestley-Taylor equation, and a modified Penman equation produced 

results that corresponded well with the energy budget method. However, in addition to measurements of air temperature, 

2 
the equations require measurements of net radiation, wind speed, and relative humidity. Because net radiation, wind speed, 

and relative humidity are not measured routinely at most climate stations, the equations cannot be used for most lake or 

reservoir evaporation studies. 

The most routinely measured variables at most climate stations are precipitation and air temperature. Because 

evaporation is directly related to air temperature, many evaporation estimation procedures use air temperature as a 

significant variable (Rosenberg and others, 1983). In Guenthner and others (1990), monthly pan evaporation data for 

Fargo, N. Dak., was regressed against monthly mean maximum and minimum air temperatures for Fargo for 1963-80. The 

resultant regression equation had a positive coefficient for the monthly mean maximum air temperature and a negative 

coefficient for the monthly mean minimum air temperature. As a result, the regression equation could produce an increase 

in monthly evaporation for a decrease in monthly mean minimum air temperature. In addition, the regression equation 

would produce the same monthly evaporation from a wide combination of monthly mean maximum and minimum air 

temperatures that could include below-freezing temperatures. A more reliable regression equation would relate monthly 

evaporation directly to only one monthly air temperature variable. The evaporation and air temperature data from the 

energy budget evaporation sites at Wetland P1 and Williams Lake are available, and the air temperature data collected from 

climate stations near Lake Ashtabula, Orwell Lake, and Lake Traverse also are available. Therefore, the combined data 

sets were used to estimate monthly evaporation from Lake Ashtabula, Orwell Lake, and Lake Traverse. 

DATA ACQUISITION 

Monthly mean air temperature data for 1931-2001 were measured at climate data stations located near the reservoir 

study sites. The data were obtained from the High Plains Regional Climate Center (2003) and from the Minnesota 

Climatology Working Group (2003). The climate data stations used near Lake Ashtabula were Cooperstown and Valley 

City, N. Dak.; the climate stations used near Orwell Lake were Campbell and Fergus Falls, Minn.; and the climate stations 

used near Lake Traverse were Beardsley and Wheaton, Minn., and Sisseton, S. Dak. (fig. 1). All of the climate data 

stations had occasional missing data during 1931-2001. Average monthly mean air temperatures for 1931-2001 were 

determined for each reservoir study site using a simple arithmetic average of the monthly mean air temperature data from 

the stations near each reservoir study site. Averaging the monthly values reduced the influence from a single station on the 

calculation of evaporation for each reservoir study site. 

Energy budget data were obtained for Wetland P1 in North Dakota and for Williams Lake in Minnesota (fig. 1) 

(D.O. Rosenberry, U.S. Geological Survey, oral commun., 2003). Data included calculated monthly evaporation and 

measured monthly mean air temperatures. Data from the two sites were recorded during May through September and 

occasionally during October for 1982-85 and 1987 at Wetland P1 and for 1982-86 at Williams Lake. Plots of monthly 

evaporation in relation to monthly mean air temperature for Wetland P1 and Williams Lake are shown in figures 2 and 3, 

respectively. 

ESTIMATION OF RESERVOIR EVAPORATION 

Monthly evaporation data sets were calculated for each reservoir study site by using regression equations of monthly 

evaporation and monthly mean air temperature for Wetland P1 and Williams Lake. Monthly evaporation data for Wetland 

P1 was regressed against the measured monthly mean air temperature data for Wetland P1, and monthly evaporation data 

for Williams Lake was regressed against the measured monthly mean air temperature data for Williams Lake. The 

regression equation for Wetland P1 was 

EP1 = 0.625T P1 + 1.218 (20 observations) (1) 

and the regression equation for Williams Lake was 

= 0.518T WL EWL – 0.008 (22 observations) (2)

3 
0 

2 

4 

6 

8 

10 

12 

14 

16 

18 

20 

r

2 = 0.64

MONTHLY EVAPORATION, IN CENTIMETERS 

0 2 4 6 8 10 12 14 16 18 20 22 24 26 


MONTHLY MEAN AIR TEMPERATURE, IN CELSIUS


Figure 2. Monthly evaporation in relation to monthly mean air temperature determined from the energy budget 

evaporation site at Wetland P1 in North Dakota, 1982-85 and 1987. 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

r

2 = 0.77

MONTHLY EVAPORATION, IN CENTIMETERS 

8 10 12 14 16 18 20 22 24 


MONTHLY MEAN AIR TEMPERATURE, IN CELSIUS


Figure 3. Monthly evaporation in relation to monthly mean air temperature determined from the energy budget 

evaporation site at Williams Lake in Minnesota, 1982-86. 

4

where

EP1	 is the monthly evaporation in centimeters at Wetland P1,


EWL is the monthly evaporation in centimeters at Williams Lake,


T P1 is the monthly mean air temperature in degrees Celsius at Wetland P1, and

T WL is the monthly mean air temperature in degrees Celsius at Williams Lake. 

For Wetland P1, the correlation coefficient was 0.80 and the root mean square error was 2.0 centimeters. For Williams 

Lake, the correlation coefficient was 0.88 and the root mean square error was 1.0 centimeters. 

The average monthly mean air temperatures determined for each reservoir study site for 1931-2001 from nearby 

climate data stations were used in place of T P1 and in equations 1 and 2 to calculate two monthly evaporation data T WL 

sets for each reservoir study site for 1931-2001. An inverse distance weighting scheme was used to determine the 

fractional contribution of each of the two monthly evaporation data sets from the energy budget evaporation sites to the 

estimated monthly evaporation data set for each reservoir study site (table 1). For example, the total distance of Lake 

Ashtabula from Wetland P1 and Williams Lake is (85 + 255) = 340 kilometers. The fractional contribution of Wetland P1 

monthly evaporation to Lake Ashtabula monthly evaporation is (340 - 85)/340 = 0.75 and the fractional contribution of 

Williams Lake monthly evaporation to Lake Ashtabula monthly evaporation is (340 - 255)/340 = 0.25. The two monthly 

evaporation data sets for each reservoir study site were multiplied by their appropriate fractional contributions and the 

results were summed to produce one weighted monthly evaporation data set for each reservoir study site. Monthly 

evaporation data during the winter period October through April often were not available for Wetland P1 and Williams 

Lake. Therefore, evaporation values for each winter month (m) and each reservoir study site (r) were estimated by using 

information presented by the Soil Conservation Service in the Hydrology Manual for North Dakota (U. S. Department of 

Agriculture, [n.d.]). Winter monthly evaporation values for each reservoir study site (E , ) were determined from the m r

average monthly mean air temperatures at each reservoir study site (T , ) by using the equation m r

(T , + 273) m r E = E ------------------------------------------	 (3) m r, m(Mean , + 273) m r

where

E is the Soil Conservation Service estimated winter monthly evaporation value, and m 

Mean is the 1971-2000 normal monthly air temperature for each reservoir study site. m r,

Temperatures in equation 3 were converted to degrees Kelvin to avoid zeros and negative values. Estimated monthly 

evaporation from Lake Ashtabula, Orwell Lake, and Lake Traverse for 1931-2001 are presented in tables 2, 3, and 4, 

respectively. 

Table 1. Distances from reservoir study sites to Wetland P1 in North Dakota and to Williams Lake in Minnesota and fractional contributions applied to 

the evaporation data sets from Wetland P1 and Williams Lake to obtain estimated monthly evaporation from Lake Ashtabula in North Dakota, 

Orwell Lake in Minnesota, and Lake Traverse in Minnesota and South Dakota 

Distances (kilometers) Fractional contributions 

Reservoir site Wetland P1 Williams Lake Wetland P1 Williams Lake 

Lake Ashtabula 85 255 0.75 0.25 

Orwell Lake 248 124 .35 .65 

Lake Traverse 238 196 .45 .55 

5 
Table 2. Estimated monthly evaporation, in centimeters, from Lake Ashtabula in North Dakota, 1931-2001 

Year January February March April May June July August September October November December Total 

1931 0.60 0.76 1.81 5.07 8.11 13.13 14.23 12.65 11.23 6.77 2.39 0.82 77.57 

1932 .59 .74 1.78 4.61 9.03 13.00 13.85 13.39 9.38 6.66 2.36 .81 76.20 

1933 .59 .73 1.81 4.60 8.59 14.25 14.02 13.10 11.21 6.68 2.37 .80 78.75 

1934 .59 .75 1.81 5.01 11.73 12.36 14.57 12.77 8.25 6.77 2.40 .80 77.81 

1935 .58 .76 1.80 4.57 6.82 10.75 15.13 12.69 9.24 6.71 2.32 .80 72.17 

1936 .56 .70 1.80 4.56 10.71 12.45 16.96 13.98 10.34 6.68 2.37 .81 81.92 

1937 .56 .73 1.79 4.58 9.26 11.13 13.95 14.56 9.67 6.69 2.36 .80 76.08 

1938 .58 .73 2.12 5.28 7.48 11.12 12.89 13.38 9.94 6.83 2.35 .81 73.51 

1939 .59 .71 1.79 4.60 10.56 10.93 14.46 13.52 9.77 6.67 2.40 .83 76.83 

1940 .58 .74 1.78 4.58 8.51 11.30 14.41 12.59 11.23 7.13 2.35 .81 76.01 

1941 .58 .73 1.79 5.53 9.89 11.86 13.94 12.99 9.38 6.72 2.38 .82 76.61 

1942 .60 .74 1.82 5.57 7.28 10.59 12.59 12.35 8.10 6.73 2.39 .80 69.56 

1943 .57 .75 1.78 5.70 7.54 11.20 14.30 13.24 9.08 6.77 2.38 .82 74.13 

1944 .60 .74 1.78 4.60 9.95 11.98 13.23 12.62 9.46 6.76 2.39 .81 74.92 

1945 .59 .74 2.14 4.59 6.51 10.01 12.91 12.79 8.80 6.73 2.36 .79 68.96 

1946 .58 .73 2.38 6.27 7.45 11.69 13.39 12.11 8.98 6.68 2.37 .81 73.44 

1947 .59 .74 1.80 4.60 7.03 10.46 13.41 14.73 9.30 7.82 2.35 .80 73.63 

1948 .58 .73 1.77 4.59 8.66 11.44 13.89 13.43 11.67 6.74 2.38 .80 76.68 

1949 .58 .73 1.79 6.23 9.31 12.10 13.86 13.76 8.95 6.73 2.41 .80 77.25 

1950 .56 .73 1.79 4.53 7.19 12.01 12.57 11.39 9.67 6.72 2.33 .79 70.28 

1951 .57 .74 1.76 4.59 9.72 9.72 12.98 11.79 8.21 6.69 2.34 .79 69.90 

1952 .58 .75 1.79 6.41 8.99 12.64 13.55 13.01 11.07 6.70 2.39 .81 78.69 

1953 .59 .75 1.81 4.57 8.23 11.68 13.30 14.07 9.73 8.11 2.41 .81 76.06 

1954 .57 .77 1.80 4.61 7.18 11.46 13.98 12.74 9.18 6.72 2.41 .82 72.24 

1955 .59 .73 1.78 7.06 10.22 11.56 14.66 14.31 9.62 6.76 2.33 .79 80.41 

1956 .58 .73 1.78 4.55 8.39 13.49 12.58 12.72 8.58 7.11 2.39 .81 73.71 

1957 .58 .74 1.81 4.61 8.75 10.90 14.77 12.68 8.54 6.72 2.38 .82 73.30 

1958 .60 .74 1.82 4.98 9.38 9.88 12.19 12.92 9.84 6.74 2.37 .80 72.26 

1959 .58 .73 1.83 4.61 7.86 12.37 13.58 13.82 9.35 6.63 2.34 .83 74.53 

1960 .58 .74 1.77 4.60 8.80 10.87 13.64 13.41 10.06 6.74 2.38 .80 74.39 

1961 .59 .75 1.83 4.57 7.81 12.68 12.76 14.37 8.52 6.74 2.38 .80 73.80 

1962 .58 .73 1.79 4.59 8.45 11.82 12.68 13.60 8.81 6.77 2.40 .81 73.03 

1963 .58 .74 1.83 4.74 8.34 13.05 14.13 13.34 10.74 8.93 2.40 .80 79.62 

1964 .59 .75 1.78 4.87 9.61 11.30 14.11 11.84 8.55 6.73 2.37 .79 73.29 

1965 .57 .73 1.77 4.60 8.25 11.20 12.88 12.11 5.81 6.75 2.37 .82 67.86 

1966 .57 .73 1.82 4.57 7.67 11.82 14.56 12.07 9.68 6.73 2.35 .80 73.37 

1967 .59 .73 1.81 4.57 7.12 11.16 13.02 12.61 10.21 6.70 2.38 .81 71.71 

1968 .58 .74 1.83 4.81 7.53 11.32 12.92 12.21 9.39 6.73 2.39 .80 71.25 

1969 .57 .74 1.77 4.86 9.13 9.53 13.03 14.49 9.89 6.64 2.39 .81 73.85 

1970 .57 .74 1.77 4.58 7.64 12.75 14.12 13.55 9.78 6.71 2.36 .80 75.37 

1971 .57 .74 1.80 5.12 8.12 12.46 12.01 13.38 9.43 6.73 2.37 .80 73.53 

1972 .58 .73 1.79 4.58 9.52 11.98 12.11 12.87 8.49 6.67 2.37 .79 72.48 

1973 .59 .75 1.82 4.60 8.41 11.75 13.15 13.81 8.45 6.77 2.35 .80 73.25 

1974 .57 .74 1.78 4.59 7.04 11.52 14.47 11.38 7.99 6.73 2.38 .82 70.01 

1975 .59 .74 1.78 4.52 8.52 11.31 14.15 12.16 8.31 6.74 2.39 .81 72.02 

1976 .58 .75 1.79 5.27 8.58 12.33 13.25 13.59 9.74 6.63 2.35 .80 75.66 

1977 .57 .75 1.82 6.48 11.77 12.06 13.33 10.27 8.70 6.73 2.36 .79 75.63 

1978 .57 .73 1.80 4.60 9.70 11.59 13.29 12.80 11.11 6.73 2.35 .79 76.06 

1979 .57 .72 1.78 4.53 6.27 11.37 13.56 11.96 10.32 6.69 2.36 .82 70.95 

1980 .58 .74 1.78 6.29 10.05 11.64 13.52 11.79 8.74 6.68 2.40 .81 75.02 

6 
Table 2. Estimated monthly evaporation, in centimeters, from Lake Ashtabula in North Dakota, 1931-2001—Continued 

Year January February March April May June July August September October November December Total 

1981 0.59 0.75 1.83 5.31 8.04 10.53 13.23 13.06 8.99 6.70 2.41 0.80 72.24 

1982 .56 .73 1.79 4.58 8.76 9.77 13.21 12.43 8.81 6.71 2.35 .82 70.52 

1983 .60 .75 1.81 4.58 7.10 11.44 14.47 14.66 9.10 6.70 2.39 .78 74.38 

1984 .59 .76 1.79 4.69 7.88 11.71 13.27 13.96 7.83 6.72 2.37 .80 72.37 

1985 .58 .74 1.83 5.62 10.03 9.93 13.10 11.27 7.86 6.68 2.31 .79 70.74 

1986 .59 .74 1.83 4.61 8.94 12.23 13.34 11.44 8.16 6.71 2.34 .82 71.75 

1987 .60 .76 1.81 6.81 10.11 12.82 13.87 11.77 9.57 6.68 2.40 .82 78.02 

1988 .58 .73 1.81 4.72 10.74 14.19 14.31 13.31 9.15 6.68 2.37 .81 79.40 

1989 .59 .73 1.78 4.61 8.95 11.11 14.73 13.32 9.52 6.72 2.36 .79 75.21 

1990 .60 .75 1.82 4.61 8.11 11.88 12.85 12.98 10.46 6.70 2.39 .80 73.95 

1991 .58 .76 1.82 5.49 9.72 12.74 13.04 13.68 9.36 6.67 2.34 .81 77.01 

1992 .59 .76 1.83 4.59 9.63 10.56 10.98 11.33 8.68 6.69 2.36 .80 68.80 

1993 .58 .73 1.80 4.60 8.36 10.08 11.65 12.37 7.96 6.67 2.36 .81 67.97 

1994 .57 .72 1.81 4.59 9.52 12.11 12.10 11.79 10.09 6.75 2.39 .81 73.25 

1995 .58 .74 1.80 4.56 7.70 12.92 12.81 13.34 9.22 6.69 2.34 .80 73.50 

1996 .57 .74 1.77 4.55 7.49 12.19 12.64 13.12 9.47 6.69 2.31 .79 72.33 

1997 .57 .74 1.78 4.54 7.33 12.70 13.09 12.06 10.12 6.72 2.35 .82 72.82 

1998 .58 .76 1.80 5.71 9.72 10.64 13.26 13.34 10.64 6.72 2.37 .81 76.35 

1999 .58 .75 1.81 4.61 8.72 11.41 13.25 12.35 8.30 6.68 2.41 .82 71.69 

2000 .58 .75 1.83 4.60 8.71 10.55 13.11 12.58 8.80 6.73 2.36 .78 71.38 

2001 .59 .73 1.80 4.61 9.04 11.33 13.51 13.38 9.32 6.67 2.85 .81 74.64 

7 
Table 3. Estimated monthly evaporation, in centimeters, from Orwell Lake in Minnesota, 1931-2001 

Year January February March April May June July August September October November December Total 

1931 0.57 0.72 1.70 4.49 6.96 12.09 13.10 11.35 10.45 6.36 2.22 0.76 70.77 

1932 .56 .70 1.67 4.43 8.49 11.87 12.73 11.99 8.28 6.27 2.19 .75 69.93 

1933 .56 .69 1.70 4.29 7.80 12.96 13.37 11.57 9.93 6.27 2.18 .75 72.07 

1934 .56 .70 1.69 4.30 10.81 11.56 12.97 11.39 7.18 6.37 2.23 .75 70.51 

1935 .55 .71 1.70 4.27 6.50 9.66 14.03 11.92 8.63 6.29 2.16 .75 67.17 

1936 .54 .67 1.69 4.25 9.69 10.98 15.24 12.86 9.81 6.28 2.18 .76 74.95 

1937 .54 .69 1.68 4.29 8.43 10.42 13.00 13.62 9.02 6.29 2.19 .75 70.92 

1938 .55 .70 1.73 4.31 6.90 10.64 12.35 13.08 9.47 6.78 2.19 .76 69.46 

1939 .56 .68 1.69 4.28 9.83 10.88 12.91 12.14 9.19 6.29 2.23 .77 71.45 

1940 .55 .70 1.68 4.28 7.64 10.40 12.90 11.34 10.23 6.54 2.18 .76 69.20 

1941 .56 .70 1.70 5.93 9.29 11.18 13.07 12.22 8.84 6.34 2.22 .77 72.82 

1942 .56 .70 1.72 5.57 6.92 10.09 11.75 11.68 7.50 6.34 2.21 .75 65.79 

1943 .54 .70 1.67 4.31 6.70 10.67 13.06 11.89 8.27 6.36 2.21 .77 67.15 

1944 .57 .70 1.68 4.28 8.75 11.27 11.87 12.16 9.38 6.38 2.23 .76 70.03 

1945 .56 .70 1.72 4.27 5.78 9.01 11.67 11.71 8.11 6.31 2.20 .74 62.78 

1946 .55 .69 1.73 5.55 6.78 10.74 12.40 11.01 8.22 6.30 2.20 .75 66.92 

1947 .56 .69 1.69 4.27 6.16 9.62 12.42 13.46 8.73 7.26 2.18 .75 67.79 

1948 .55 .69 1.67 4.35 7.93 10.19 12.63 11.93 10.44 6.34 2.21 .75 69.68 

1949 .55 .69 1.69 4.66 8.58 11.04 12.55 12.84 7.75 6.35 2.23 .75 69.68 

1950 .54 .69 1.67 4.22 6.38 10.90 11.17 10.54 9.05 6.35 2.18 .74 64.43 

1951 .54 .70 1.65 4.27 8.64 9.28 11.65 10.70 7.26 6.30 2.17 .74 63.90 

1952 .55 .70 1.67 4.71 8.06 11.34 12.28 11.53 9.50 6.27 2.22 .76 69.59 

1953 .56 .70 1.70 4.27 7.60 11.01 12.22 12.59 8.78 7.05 2.23 .75 69.46 

1954 .54 .72 1.69 4.29 6.21 10.84 12.59 11.55 8.19 6.32 2.23 .76 65.93 

1955 .56 .69 1.67 6.48 9.54 10.79 13.50 12.91 8.68 6.35 2.17 .74 74.08 

1956 .55 .69 1.68 4.25 7.64 12.21 11.52 11.65 7.86 6.70 2.21 .76 67.72 

1957 .55 .70 1.70 4.30 7.53 10.15 13.64 11.67 8.04 6.32 2.21 .76 67.57 

1958 .56 .70 1.70 4.59 8.86 8.88 11.21 11.92 9.08 6.35 2.21 .75 66.81 

1959 .55 .69 1.71 4.31 8.24 11.70 12.70 13.06 9.03 6.25 2.17 .77 71.18 

1960 .55 .70 1.66 4.29 8.00 10.13 12.70 12.46 9.41 6.34 2.21 .75 69.20 

1961 .55 .71 1.72 4.25 6.77 11.51 12.02 12.86 8.48 6.34 2.21 .75 68.17 

1962 .55 .69 1.68 4.27 8.11 10.55 11.37 11.78 7.73 6.37 2.23 .76 66.09 

1963 .54 .70 1.71 4.54 7.43 11.55 12.73 11.71 9.52 7.96 2.23 .74 71.36 

1964 .56 .71 1.67 4.31 9.08 10.94 13.13 10.78 7.73 6.32 2.21 .74 68.18 

1965 .54 .69 1.66 4.29 7.97 10.56 11.86 11.19 5.63 6.34 2.20 .77 63.70 

1966 .54 .69 1.71 4.26 6.82 10.90 13.51 11.06 8.79 6.30 2.18 .75 67.51 

1967 .55 .69 1.70 4.30 6.25 10.26 11.51 11.15 9.02 6.30 2.21 .76 64.70 

1968 .55 .69 1.72 4.38 6.61 10.56 11.97 11.57 8.68 6.32 2.21 .75 66.01 

1969 .54 .70 1.66 4.31 7.97 8.48 11.65 12.73 9.09 6.27 2.21 .76 66.37 

1970 .54 .70 1.67 4.28 7.39 11.57 13.07 12.50 9.15 6.33 2.20 .75 70.15 

1971 .54 .70 1.69 4.30 7.18 11.69 10.84 11.75 8.91 6.35 2.21 .75 66.91 

1972 .54 .69 1.69 4.29 8.82 11.00 11.51 11.95 7.90 6.28 2.20 .74 67.61 

1973 .56 .71 2.07 4.30 7.69 11.19 12.23 12.99 8.36 6.74 2.21 .75 69.80 

1974 .55 .70 1.69 4.31 6.72 10.75 13.62 10.91 7.48 6.35 2.21 .77 66.06 

1975 .56 .69 1.67 4.24 8.38 10.79 13.37 11.89 7.78 6.36 2.22 .76 68.71 

1976 .55 .71 1.69 5.65 7.94 12.14 13.06 13.39 9.22 6.24 2.18 .74 73.51 

1977 .54 .70 1.71 5.48 10.74 10.77 12.36 9.59 8.17 6.30 2.18 .74 69.28 

1978 .54 .68 1.68 4.27 8.39 10.40 11.84 11.84 10.16 6.32 2.19 .74 69.05 

1979 .54 .68 1.67 4.23 6.28 10.60 12.09 10.71 9.55 6.30 2.20 .77 65.62 

1980 .55 .70 1.68 5.59 9.44 10.84 12.56 11.55 8.44 6.29 2.23 .75 70.62 

8 
Table 3. Estimated monthly evaporation, in centimeters, from Orwell Lake in Minnesota, 1931-2001—Continued 

Year January February March April May June July August September October November December Total 

1981 0.56 0.71 1.72 4.92 7.80 10.10 12.35 11.60 8.58 6.30 2.24 0.75 67.63 

1982 .54 .69 1.68 4.28 8.72 9.08 12.43 11.69 8.07 6.31 2.19 .76 66.44 

1983 .56 .71 1.71 4.28 6.69 10.52 13.27 13.24 8.61 6.29 2.22 .73 68.83 

1984 .55 .72 1.68 4.60 7.02 10.69 12.26 12.64 7.07 6.34 2.21 .75 66.53 

1985 .55 .69 1.72 5.13 9.36 9.17 11.88 10.40 7.70 6.30 2.15 .74 65.79 

1986 .56 .69 1.70 4.31 8.07 11.36 12.34 10.47 7.81 6.32 2.18 .76 66.57 

1987 .56 .72 1.71 6.43 9.35 11.80 12.75 11.14 8.92 6.27 2.23 .76 72.64 

1988 .55 .69 1.70 4.31 10.19 13.10 13.61 12.72 8.82 6.27 2.20 .75 74.91 

1989 .56 .68 1.67 4.29 8.20 10.37 13.61 11.96 8.69 6.33 2.18 .74 69.28 

1990 .57 .70 1.71 4.31 7.18 10.92 11.84 11.88 9.62 6.30 2.22 .75 68.00 

1991 .55 .71 1.70 5.02 9.24 12.04 12.02 12.54 8.22 6.28 2.17 .76 71.25 

1992 .56 .71 1.71 4.27 8.78 9.89 10.01 10.30 8.05 6.30 2.19 .75 63.52 

1993 .55 .69 1.68 4.29 7.72 9.51 11.02 11.58 6.86 6.28 2.18 .75 63.11 

1994 .54 .68 1.70 4.30 8.82 11.39 11.38 10.85 9.56 6.35 2.22 .76 68.55 

1995 .55 .70 1.69 4.26 7.29 12.05 12.05 12.31 8.61 6.30 2.17 .76 68.74 

1996 .54 .70 1.66 4.26 7.18 11.48 11.92 12.36 9.05 6.33 2.16 .74 68.38 

1997 .55 .70 1.68 4.27 6.61 12.09 11.75 11.01 9.57 6.34 2.18 .77 67.52 

1998 .56 .72 1.69 5.27 9.17 9.48 12.01 12.36 10.56 6.34 2.21 .76 71.13 

1999 .55 .71 1.71 4.31 8.52 10.90 12.55 12.01 8.14 6.30 2.66 .77 69.13 

2000 .55 .71 1.87 4.30 8.54 10.06 12.42 12.09 8.75 6.35 2.19 .73 68.56 

2001 .56 .68 1.68 4.30 8.80 11.06 12.91 12.63 8.71 6.30 3.41 .76 71.80 

9 
Table 4. Estimated monthly evaporation, in centimeters, from Lake Traverse in Minnesota and South Dakota, 1931-2001 

Year January February March April May June July August September October November December Total 

1931 0.61 0.77 1.81 5.53 7.86 13.21 14.30 12.35 11.80 6.92 2.41 0.82 78.39 

1932 .59 .75 1.78 4.69 9.09 12.42 13.61 13.01 9.35 6.67 2.37 .81 75.14 

1933 .60 .74 1.81 4.40 8.64 14.33 14.63 12.80 11.39 6.70 2.38 .81 79.23 

1934 .60 .75 1.80 5.03 11.82 12.61 13.90 12.18 8.11 6.99 2.41 .81 77.01 

1935 .59 .77 1.82 4.05 6.92 10.36 14.56 12.75 9.53 6.70 2.35 .81 71.21 

1936 .57 .71 1.81 4.05 10.56 11.97 16.11 13.75 10.88 6.68 2.37 .82 80.28 

1937 .58 .74 1.80 4.07 9.23 11.02 13.57 14.56 10.07 6.69 2.37 .81 75.51 

1938 .59 .75 1.83 5.27 7.69 11.58 13.05 14.12 10.35 7.82 2.37 .82 76.24 

1939 .60 .73 1.81 4.07 10.77 11.79 13.53 12.87 10.53 6.69 2.42 .83 76.64 

1940 .58 .75 1.79 4.06 8.51 11.56 14.34 11.95 11.10 7.44 2.36 .82 75.26 

1941 .60 .75 1.81 6.09 10.40 11.83 13.79 12.87 10.04 6.72 2.40 .82 78.12 

1942 .60 .75 1.83 6.38 7.42 10.67 12.43 12.52 8.32 6.73 2.39 .80 70.84 

1943 .58 .75 1.78 5.38 7.32 11.24 13.78 12.83 8.75 6.74 2.38 .82 72.35 

1944 .61 .75 1.79 4.06 9.44 11.78 12.76 12.40 9.48 6.76 2.40 .82 73.05 

1945 .60 .75 2.64 4.07 6.98 9.82 12.65 12.75 9.12 6.72 2.37 .80 69.27 

1946 .59 .74 2.45 6.52 7.70 11.87 13.34 12.19 9.22 6.71 2.39 .81 74.53 

1947 .61 .74 1.80 4.06 7.30 10.64 13.39 14.83 10.26 8.30 2.36 .81 75.10 

1948 .59 .74 1.78 5.62 8.71 11.13 13.47 12.85 11.49 6.74 2.39 .81 76.32 

1949 .59 .74 1.80 5.63 9.64 12.02 13.46 13.98 8.86 6.74 2.43 .81 76.70 

1950 .58 .74 1.79 4.01 7.28 11.88 12.37 11.29 9.68 6.75 2.36 .80 69.53 

1951 .58 .75 1.76 4.06 9.32 9.93 12.56 11.55 8.16 6.70 2.36 .80 68.53 

1952 .59 .75 1.78 5.51 8.59 12.38 13.37 12.83 10.88 6.69 2.40 .82 76.59 

1953 .60 .75 1.81 4.05 8.04 11.84 12.77 13.20 9.88 8.15 2.42 .82 74.33 

1954 .58 .77 1.80 4.58 6.75 11.38 13.41 12.46 9.15 6.70 2.47 .82 70.87 

1955 .59 .74 1.79 7.31 10.44 11.30 14.35 13.84 9.77 6.76 2.34 .80 80.03 

1956 .59 .74 1.79 4.04 8.47 12.98 12.14 12.34 9.07 7.76 2.39 .82 73.13 

1957 .59 .75 1.81 4.69 8.01 10.99 14.43 12.42 9.04 6.72 2.39 .83 72.67 

1958 .60 .75 1.81 5.18 9.68 10.02 12.53 13.51 10.56 6.97 2.40 .81 74.82 

1959 .59 .74 1.83 5.06 8.98 12.85 13.68 14.06 10.17 6.65 2.35 .83 77.79 

1960 .59 .75 1.77 4.43 8.75 10.74 13.67 13.32 10.42 6.75 2.40 .81 74.40 

1961 .59 .76 2.02 4.05 7.76 12.68 12.63 13.90 9.07 6.75 2.39 .80 73.40 

1962 .59 .74 1.79 4.07 9.06 11.35 12.02 12.81 8.91 6.99 2.41 .82 71.56 

1963 .58 .75 1.83 5.37 8.38 12.40 13.26 12.61 10.51 8.95 2.41 .80 77.85 

1964 .60 .76 1.79 5.18 9.83 12.02 14.20 12.01 8.90 6.75 2.39 .80 75.23 

1965 .58 .74 1.77 4.31 8.86 11.40 12.87 12.35 6.50 7.01 2.39 .83 69.61 

1966 .58 .74 1.83 4.05 7.57 11.52 14.74 11.80 9.55 6.73 2.36 .81 72.28 

1967 .59 .74 1.82 4.30 6.92 11.05 12.47 12.03 10.01 6.71 2.39 .82 69.85 

1968 .59 .74 3.22 5.26 7.36 11.41 12.94 12.69 9.64 6.73 2.40 .81 73.79 

1969 .58 .75 1.77 5.13 8.95 9.51 12.68 13.91 10.19 6.66 2.40 .82 73.35 

1970 .58 .75 1.79 4.07 8.54 12.44 13.87 13.23 10.25 6.73 2.38 .81 75.44 

1971 .58 .75 1.81 5.62 8.13 12.47 11.99 12.96 9.93 6.75 2.39 .81 74.19 

1972 .59 .74 1.80 4.07 9.52 11.73 12.64 13.11 9.30 6.68 2.38 .80 73.36 

1973 .60 .76 2.65 4.71 8.47 12.20 13.07 13.90 9.08 7.61 2.38 .81 76.24 

1974 .59 .75 1.80 4.78 7.65 11.50 14.73 12.06 8.85 6.80 2.40 .82 72.73 

1975 .60 .74 1.78 4.02 9.17 11.58 14.60 12.65 8.72 6.77 2.40 .81 73.84 

1976 .59 .77 1.81 6.36 8.64 12.92 14.24 14.41 10.46 6.67 2.37 .81 80.05 

1977 .58 .76 1.83 7.16 11.33 11.95 13.77 11.17 9.43 6.73 2.37 .80 77.88 

1978 .58 .74 1.80 4.07 9.38 11.58 13.03 13.12 11.53 6.74 2.37 .81 75.75 

1979 .58 .73 1.79 4.03 7.03 11.37 12.82 11.80 10.57 6.70 2.38 .83 70.63 

1980 .59 .74 1.79 6.28 10.05 11.52 13.45 12.18 9.53 6.70 2.41 .81 76.05 

10 
Table 4. Estimated monthly evaporation, in centimeters, from Lake Traverse in Minnesota and South Dakota, 1931-2001—Continued 

Year January February March April May June July August September October November December Total 

1981 0.60 0.76 2.16 6.35 8.69 10.95 13.13 12.71 9.74 6.71 2.46 0.81 75.07 

1982 .57 .74 1.80 4.07 9.14 10.07 13.82 12.90 9.29 6.72 2.37 .82 72.31 

1983 .60 .76 1.81 4.06 7.84 11.68 14.72 14.62 10.04 6.72 2.39 .79 76.03 

1984 .60 .77 1.79 5.00 8.54 11.82 13.41 13.85 8.36 6.74 2.40 .81 74.09 

1985 .59 .75 1.83 6.60 10.32 10.50 13.41 11.53 8.52 6.71 2.33 .80 73.89 

1986 .60 .74 1.83 5.10 9.11 12.31 13.62 11.55 8.78 6.73 2.37 .83 73.57 

1987 .61 .77 1.83 7.41 10.20 12.61 14.19 12.26 10.11 6.68 2.41 .82 79.90 

1988 .59 .74 1.82 5.47 10.89 13.94 14.71 13.54 9.74 6.68 2.38 .82 81.32 

1989 .60 .73 1.79 4.48 9.05 11.21 14.09 12.57 9.65 6.73 2.37 .80 74.07 

1990 .61 .76 1.83 5.00 8.07 11.87 12.51 12.58 10.59 6.71 2.41 .81 73.75 

1991 .59 .76 1.82 5.91 9.69 12.57 12.90 13.27 9.57 6.70 2.35 .82 76.95 

1992 .61 .77 1.83 4.07 9.72 10.62 10.87 11.28 9.44 6.71 2.37 .81 69.10 

1993 .59 .74 1.80 4.24 8.49 10.16 11.82 12.31 8.20 6.71 2.37 .81 68.24 

1994 .58 .74 1.82 5.18 9.77 12.28 12.18 11.62 10.46 6.76 2.41 .82 74.62 

1995 .59 .75 1.81 4.04 8.08 12.49 12.89 13.45 9.33 6.69 2.36 .81 73.29 

1996 .58 .75 1.78 4.05 7.53 12.08 12.49 12.76 9.37 6.72 2.33 .80 71.24 

1997 .58 .75 1.79 4.05 7.71 12.54 12.91 12.21 10.73 6.75 2.37 .83 73.22 

1998 .59 .77 1.80 6.05 10.20 10.91 13.42 13.39 11.35 6.75 2.40 .82 78.45 

1999 .59 .77 1.83 5.12 9.13 11.65 13.90 12.78 8.98 6.71 3.40 .83 75.69 

2000 .59 .76 2.67 4.70 9.32 11.21 13.05 12.98 9.81 6.75 2.37 .79 75.00 

2001 .60 .74 1.79 4.53 9.38 12.02 14.00 13.42 9.70 6.71 4.15 .82 77.86 

Results from the procedure that estimates reservoir evaporation indicate that either Lake Ashtabula or Lake Traverse 

had the most evaporation per year, whereas Orwell Lake had somewhat less (fig. 4). Regression analyses indicate that 

slight downward trends in annual evaporation (about -0.05 centimeters per year) occurred from 1931-2001 at all three 

reservoir study sites. The trends may have been caused by the selected time period of the study, which began with the 

drought conditions in the mid 1930’s and ended with the more wet conditions in the late 1990’s. Average annual 

evaporation for each reservoir for 1931-2001 was about 74.0 centimeters from Lake Ashtabula, about 68.2 centimeters 

from Orwell Lake, and about 74.6 centimeters from Lake Traverse. These values correspond well with average annual lake 

evaporation values for 1946-55 (Kohler and others, 1959). 

METHOD CONSIDERATIONS AND LIMITATIONS 

The data used in this report were obtained from rigorous methods and established climate stations; however, the data 

still only represent conditions at a point or small area. It is possible that physical and atmospheric conditions at the 

reservoir study sites and at the energy budget evaporation sites may have been inadequately represented by the data. The 

use of monthly values can improve representation of conditions by integrating and smoothing point and daily information 

into a single value. 

Regression equations usually cannot be used to indicate cause and effect. However, because the data from the two 

energy budget evaporation sites were determined from a rigorous energy budget method, and because air temperature 

directly influences evaporation from open-water surfaces, the results from the regression equations could be considered 

cause and effect. A limitation of the method is that the evaporation data from the two energy budget evaporation sites in 

North Dakota and Minnesota covered a time period that did not include the most recent dry episode in the late 1980’s and 

early 1990’s. Data from the dry episode may have indicated whether the relation between monthly evaporation and 

monthly mean air temperature was different than the linear function used in the regressions. In addition, the monthly 

evaporation corrections used for the winter months October through April were general corrections from other published 

material and were not based on specific factors associated with each reservoir. 

11 
85 

Lake Ashtabula 

Orwell Lake 

Lake Traverse 

65 

70 

75 

80 

ANNUAL EVAPORATION, IN CENTIMETERS 

60 

1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 

Figure 4. Annual evaporation determined from the regression equation for Lake Ashtabula in North Dakota, Orwell Lake in 

Minnesota, and Lake Traverse in Minnesota and South Dakota, 1931-2001. 

SUMMARY 

Reliable water supplies are needed for population growth of cities in the Red River of the North Basin. Currently, 

reservoirs on tributaries of the Red River of the North provide water for Fargo and Grand Forks, N. Dak., and other cities 

along the river. The Dakota Water Resources Act, passed by the U. S. Congress on December 15, 2000, authorized the 

Secretary of the Interior to conduct a comprehensive study of the future water-quantity and quality needs of the Red River 

of the North Basin in North Dakota and the possible options to meet those water needs. Adequate estimates of evaporative 

losses from reservoirs in the Red River of the North Basin are needed to determine the total water supply in the Basin. A 

study was conducted by the U.S. Geological Survey in cooperation with the Bureau of Reclamation to review studies that 

evaluated evaporation estimation methods and to estimate monthly evaporation from Lake Ashtabula in North Dakota, 

Orwell Lake in Minnesota, and Lake Traverse in Minnesota and South Dakota for 1931-2001. 

Pan evaporation data commonly are used to estimate evaporation from lakes and reservoirs; however, there are 

problems with using these data. Debris in the water, animal activity in and around the pan, size of the pan, materials used 

to construct the pan, exposure of the pan, strong winds, and measurement of water depth in the pan can introduce errors in 

pan evaporation rates. The energy budget method was used to determine evaporation from a wetland in North Dakota, 

Wetland P1, and from a lake in Minnesota, Williams Lake. The energy budget method relies on the optimum placement of 

sensors in and around the lake or reservoir to provide a rigorous estimate of evaporation. The energy budget method was 

12 
compared to 11 other evaporation equations. Of the 11 equations evaluated, 3 equations produced results that 

corresponded well with the energy budget method. However, in addition to measurements of air temperature, the equations 

require measurements of net radiation, wind speed, and relative humidity. The evaporation and air temperature data from 

the energy budget evaporation sites at Wetland P1 and Williams Lake are available, and the air temperature data collected 

from climate stations near Lake Ashtabula, Orwell Lake, and Lake Traverse also are available. Therefore, the combined 

data sets were used to estimate monthly evaporation from Lake Ashtabula, Orwell Lake, and Lake Traverse. 

Monthly mean air temperature data for 1931-2001 were collected from climate data stations located near the reservoir 

study sites. Energy budget data were obtained for Wetland P1 and Williams Lake. Monthly evaporation data for Wetland 

P1 was regressed against the measured monthly mean air temperature data for Wetland P1, and monthly evaporation data 

for Williams Lake was regressed against the measured monthly mean air temperature data for Williams Lake. Averaged 

monthly mean air temperatures determined for each reservoir study site for 1931-2001 from nearby climate data stations 

were used in the Wetland P1 and Williams Lake equations to calculate two monthly evaporation data sets for each reservoir 

study site for 1931-2001. An inverse distance weighting scheme was used to determine the fractional contribution of each 

of the two monthly evaporation data sets from the energy budget evaporation sites to the estimated monthly evaporation 

data set for each reservoir study site. The two monthly evaporation data sets for each reservoir study site were multiplied 

by their appropriate fractional contributions and the results were summed to produce one weighted monthly evaporation 

data set for each reservoir study site. 

Results from the procedure that estimates reservoir evaporation indicate that slight downward trends in annual 

evaporation occurred from 1931-2001. The trends may have been caused by the selected time period of the study, which 

began with the drought conditions in the mid 1930’s and ended with the more wet conditions in the late 1990’s. Average 

annual evaporation values for each reservoir for 1931-2001 correspond well with published average annual lake 

evaporation values for 1946-55. 

REFERENCES 

Farnsworth, R.K., Thompson, E.S., and Peck, E.L., 1982, Evaporation atlas for the contiguous 48 United States: U.S. Department of 

Commerce National Oceanic and Atmospheric Administration, National Weather Service, NOAA Technical Report NWS 33, 

Washington, D.C., 26 p. 

Garrison Diversion Conservancy District and the Dakotas Area Office of the Bureau of Reclamation, [n.d.], Dakota Water Resources 

Act of 2000: accessed July 2003 at URL htp://www.rrvwsp.com/dwra.htm 

Guenthner, R.S., Weigel, J.F., and Emerson, D.G., 1990, Gaged and estimated monthly streamflow during 1931-84 for selected sites in 

the Red River of the North Basin in North Dakota and Minnesota: U.S. Geological Survey Water-Resources Investigations Report 

90-4167, 230 p. 

High Plains Regional Climate Center, 2003, Climate Data - Holdings & Services: accessed February 2003 at URL 

http://www.hprcc.unl.edu/index.html 

Kohler, M.A., Nordenson, T.J., and Baker, D.R., 1959, Evaporation maps for the United States: U.S. Department of Commerce 

Weather Bureau, Technical Paper No. 37, Washington, D.C., 13 p. 

Minnesota Climatology Working Group, 2003, Historical Climate Data Retrieval: accessed February 2003 at URL 

http://climate.umn.edu/doc/historical.htm 

Parkhurst, R.S., Winter, T.C., Rosenberry, D.O., and Sturrock, A.M., 1998, Evaporation from a small prairie wetland in the Cottonwood 

Lake area, North Dakota - an energy budget study: Wetlands, v. 18, no. 2, p. 272-287. 

Rosenberg, N.J., Blad, B.L., and Verma, S.B., 1983, Microclimate--the biological environment (2nd ed.): John Wiley and Sons, New 

York, 495 p. 

Rosenberry, D.O., Sturrock, A.M., and Winter, T.C., 1993, Evaluation of the energy budget method of determining evaporation at 

Williams Lake, Minnesota, using alternative instrumentation and study approaches: Water Resources Research, v. 29, no. 8, 

p. 2473-2483. 

13 
U.S. Department of Agriculture, [n.d.], Hydrology manual for North Dakota: 	U.S. Department of Agriculture, Soil Conservation 

Service, Bismarck, N. Dak., 230 p. 

Winter, T.C., Rosenberry, D.O., and Sturrock, A.M., 1995, Evaluation of 11 equations for determining evaporation for a small lake in 

the north central United States: Water Resources Research, v. 31, no. 4, p. 983-993. 

14